Vinyl-terminated polymers, including for the purposes of this application oligomers, homopolymers and copolymers synthesized from two or more monomers, are known to be useful for post-polymerization (or post-oligomerization) reactions due to the available ethylenic unsaturation in one polymer, at one chain end, or both. Such reactions include addition reactions, such as those used in grafting other ethylenically unsaturated moieties, and further insertion polymerization where the vinyl-terminated polymers are copolymerized with other monomers such as .alpha.-olefins and/or other insertion polymerizable monomers. In this latter instance the vinyl-terminated polymers are often called macromonomers, or macromers.
Early work with metallocene transition metal catalyst compounds activated with alkylalumoxanes such as methylalumoxane led to observations that their use in olefin polymerization gave rise to unsaturated end-groups in a greater percentage of polymer produced than had typically been true of insertion polymerization using traditional, pre-metallocene Ziegler-Natta catalysts. See EP-A-0 129 638 and its U.S. Pat. No. 5,324,800. Later work by Resconi, et al., reported in Olefin Polymerization at Bis(pentamethylcyclopentadienyl)zirconium and --hafnium centers: Chain-Transfer Mechanisms, J. Am. Chem. Soc., 1992, 114, 1025-1032, yielded the observations that the use of bis(pentamethylcyclopentadienyl) zirconcene or hafnocene in propylene oligomerization favors .beta.-methyl elimination over the more commonly expected .beta.-hydride elimination as the means for chain transfer, or polymer chain termination. This was based on observations that the ratio of vinyl-end groups to vinylidene-end groups was in the range of 92 to 8 for the zirconocene and 98 to 2 for the hafnocene. The polymerization of propylene in this article yielded atactic propylene oligomers and low molecular weight polymers. Similar results have been achieved by Shiono, et al., reported in Copolymerization of poly(propylene) macromonomer and ethylene with metallocene catalysts, Macromol. Symp. 97, 161-170 (1995), and Yang, et al., reported in Cationic Zirconocene Olefin Polymerization Catalysts Based on the Organo-Lewis Acid Tris(pentafluorophenyl)borane. A Synthetic, Structural, Solution Dynamic, and Polymerization Catalytic Study, J. Am. Chem. Soc., 1994, 116, 10015-10031.
In addition to these observations, WO 94/07930 addresses advantages of including long chain branches in polyethylene from incorporating vinyl-terminated macromers into polyethylene chains where the macromers have critical molecular weights greater than 3,800, or, in other words contain 250 or more carbon atoms. Conditions said to favor the formation of vinyl terminated polymers are high temperatures, no comonomer, no transfer agents, and a non-solution process or a dispersion using an alkane diluent. Increase of temperature during polymerization is also said to yield .beta.-hydride eliminated product, for example while adding ethylene so as to form an ethylene "end cap". This document goes on to describe a large class of both mono-cyclopentadienyl and bis-cyclopentadienyl metallocenes as suitable in accordance with the invention when activated by either alumoxanes or ionizing compounds providing stabilizing, noncoordinating anions. The examples all illustrate the use of the Lewis acid activator tris(perfluorophenyl) boron with bis(cyclopentadienyl) zirconium dimethyl at a polymerization temperature of 90.degree. C. Copolymerization was conducted with ethylene and the two macromers, respectively, using the same catalyst systems as used to form the macromers.
Additional art addresses the preparation of chain-end unsaturated polymers with various metallocenes under various conditions, each of vinyl-, vinylidene-, vinylene- and trisubstituted-unsaturation resulting from the reported processes. The difficulty in determining by standard characterization methods (.sup.1 H-NM or .sup.13 C-NMR) the total of saturated chain ends has resulted in acceptance in the art of characterizing unsaturated end-group by the fraction of the total of each type of unsaturation to the total unsaturated ends. However, industrially efficient methods of production would greatly benefit from high unsaturated end group concentrations to the total end group population, that is including the saturated ends. Thus, the reported variations in molecular weight distributions and the inability to accurately determine or predict the resulting type of chain ends, or the less favored production of unsaturated chain-ends other than those of vinyl, limits the utility of the prior art.
Vinyl-chain ends are generally accepted to be more reactive to chain-end functionalization and insertion in subsequent polymerization reactions than are the other types and are more highly preferred. Therefore, polypropylene macromers with a high percentage of vinyl terminal bonds would be desirable for use in the preparation of branched polymers. In addition, stereospecific polypropylene (i.e. isotactic and/or syndiotactic polypropylene) is more desirable than atactic polypropylene. Stereospecific polypropylene has a more crystalline structure which imparts greater strength properties to the polymer. Accordingly, a need still exists for polypropylene macromers which are stereospecific to impart strength and have a high percentage of vinyl terminal bonds for improved utility in the preparation of branched polymers.